1. Field of the Invention
The invention relates to a semiconductor memory device and a method of fabricating the same, and more particularly to a semiconductor memory device having a cylindrical accumulation electrode and a method of fabricating the same.
2. Description of the Related Art
Among memory cells used for a highly integrated semiconductor memory or DRAM, a memory cell comprised of a single transistor and a single capacitor has the small number of constituents and can be readily reduced in an area, and hence, has been widely employed.
Since an output voltage produced by a memory cell is in proportion to a capacity of a capacitor included in a memory cell, it would be necessary for the capacitor to have a sufficient capacity in order to ensure stable operation of the memory cell, even when the memory cell is designed to be small-sized and highly integrated. As a capacitor designed to have a sufficiently great capacity is known a capacitor including a cylindrical accumulation electrode.
For instance, such a capacitor including a cylindrical accumulation electrode is disclosed in Japanese Unexamined Patent Publications Nos. 5-218332, 6-151747, 8-153858, and 8-316435.
FIGS. 1 to 4 are cross-sectional views of a memory cell including a cylindrical accumulation electrode, illustrating respective steps of a method of fabricating a cylindrical accumulation electrode, disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 5-218332.
The method of a fabricating a cylindrical accumulation electrode is explained hereinbelow with reference to FIGS. 1 to 4.
First, as illustrated in FIG. 1, a transistor and a capacitor is fabricated on a semiconductor substrate 10 in accordance with a conventional method of fabricating a semiconductor device. At this stage, a resultant semiconductor device is covered at a surface thereof with a lower interlayer insulating film 11.
Then, a nitride film 12 and an oxide film 13 are successively deposited on the lower interlayer insulating film 11. The nitride film 12 and the oxide film 13 cooperate with each other to thereby form an insulating film 14. Then, as illustrated in FIG. 2, openings 15 are formed throughout the insulating film 14 in areas where accumulation electrodes are to be fabricated. The openings 15 reach the lower interlayer insulating film 11.
Then, as illustrated in FIG. 3, a thin polysilicon film 16 is formed on an inner sidewall and a bottom of each of the openings 15. The thin polysilicon film 16 is designed to have such a thickness that the thickness is sufficient as a sidewall of a cylindrical accumulation electrode and that the openings 15 are not filled with the polysilicon film 16. By designing the polysilicon film 16 to have such a thickness, there are formed recesses 16a surrounded by the polysilicon film 16.
Then, as illustrated in FIG. 3, the recesses 16a are filled with oxide films 17.
Then, a portion 16b (illustrated with broken lines) of the polysilicon film 16 formed on the oxide film 13 is etched back by dry etching for removal. Thereafter, the oxide film 13 is removed by etching.
Thus, there is obtained a cylindrical accumulation electrode 18. Then, as illustrated in FIG. 4, the cylindrical accumulation electrode 18 is entirely covered with a capacity plate or an opposing electrode 19. In the thus fabricated cylindrical accumulation electrode 18, a capacity C is defined by the capacity plate 19 and an outer surface 20 of a sidewall of the cylindrical accumulation electrode 18, acting as a capacity film.
In accordance with the cylindrical accumulation electrode 18, an outer surface and an inner surface of a cylinder are effective areas for defining a capacity C. Hence, a capacity C could be increased relatively to an accumulation electrode in the form of a rectangular parallelopiped block.
However, the conventional cylindrical accumulation electrode 18 is accompanied with a problem of structural stability in a sidewall of a cylinder. That is, the sidewall of the cylinder might be fallen down in a process carried out subsequent to the formation of the cylinder. Such structural instability of the cylinder is a hindrance to smoothly carrying out a process of fabricating a semiconductor memory device.
To the contrary, if the cylindrical accumulation electrode 18 is designed to have a sidewall having a great thickness for ensuring structural stability, there would be paused another problem that an area defined within an inner surface of a sidewall of the cylinder cannot avoid from being small, resulting in a smaller capacity.
The reason why the above-mentioned problems are caused is that the conventional cylindrical accumulation electrode 18 is designed to have a sidewall and a bottom which have the same thickness. Specifically, since the polysilicon film 16 is formed in the opening 15 in a single step in the conventional method of fabricating the cylindrical accumulation electrode 18, a thickness of a sidewall of the cylinder is unavoidably always the same as a thickness of a bottom of the cylinder.
As a result, when an area of a memory cell is to be reduced, a thickness of a sidewall of the cylinder had to be reduced in the conventional cylindrical accumulation electrode so that opposing portions of a sidewall do not make contact with each other. Since a thickness of a sidewall is always equal to a thickness of a bottom in the conventional cylindrical accumulation electrode, if a thickness of a sidewall is reduced, a thickness of a bottom is also reduced, resulting in structural instability of a cylindrical accumulation electrode.
In particular, since a thickness of a portion through which a sidewall is connected to a bottom is also reduced, the sidewall is likely to fall down from the bottom at the portion.
In view of the above-mentioned problems in the conventional cylindrical accumulation electrode, it is an object of the present invention to provide a semiconductor memory device and a method of fabricating the same, both of which are capable of independently determining thicknesses of a sidewall and a bottom of a cylindrical accumulation electrode, and differentiating the thicknesses from each other.
In one aspect of the present invention, there is provided a semiconductor memory device including (a) a memory cell including a transistor and a capacitor, and (b) a cylindrical accumulation electrode constituted of a sidewall and a bottom, a first thickness of the sidewall and a second thickness of the bottom of the cylindrical accumulation electrode being different from each other.
There is further provided a semiconductor memory device including (a) a memory cell including a transistor and a capacitor, and (b) a cylindrical accumulation electrode constituted of a sidewall and a bottom, a first thickness of the sidewall being smaller than a second thickness of the bottom.
In the semiconductor memory device, a bottom of a cylindrical accumulation electrode is designed to have a greater thickness than a thickness of a sidewall of the cylindrical accumulation electrode. This ensures much reduction in probability in fall-down of a sidewall, and enhances structural stability in a sidewall of a cylindrical accumulation electrode. In addition, it would be possible to accomplish a semiconductor memory device capable of readily increasing a capacity thereof.
The reason why a bottom of a cylindrical accumulation electrode can have a greater thickness than a thickness of a sidewall of the cylindrical accumulation electrode is that a bottom and a sidewall of a cylindrical accumulation electrode are independently formed in accordance with the present invention, and hence, thicknesses of a bottom and a sidewall can be independently determined, unlike the conventional cylindrical accumulation electrode.
It is preferable that the first thickness is greater at a location closer to the bottom. Namely, a sidewall of the cylindrical accumulation electrode may be tapered. For instance, the sidewall has a straight outer surface and a tapered inner surface.
By designing the first thickness in the above-mentioned manner, a thickness at a lowermost portion of the sidewall is maximum. Hence, it is possible to make a portion through which the sidewall is connected to the bottom greater in size, and hence, increase strength of a connection between the sidewall and the bottom. As a result, it is possible to eliminate structural instability in a cylinder by which the sidewall may be fallen down at a portion through which the sidewall is connected to the bottom.
It is preferable that an outer surface of the sidewall is entirely exposed to a capacity plate formed around the sidewall.
In comparison with a structure in which a second insulating film remains on the lower interlayer insulating film, it is possible to increase an area of an outer surface of the sidewall, as a capacity film, ensuring an increase in a capacity.
In another aspect of the present invention, there is provided a method of fabricating a semiconductor memory device including a memory cell having transistor and a capacitor, and a cylindrical accumulation electrode, the method including the steps of (a) forming a first insulating film on a lower interlayer insulating film, (b) forming at least one hole through the first insulating film so that the hole reaches the lower interlayer insulating film, (c) forming a polysilicon layer in the hole so that an upper surface of the polysilicon layer is located lower than an upper surface of the first insulating film, (d) covering the first insulating film and the polysilicon layer with a second insulating film, (e) etching back the second insulating film so that the second insulating film remains only on a sidewall of the first insulating film, and (f) etching the polysilicon layer with the second insulating film being used as a mask so that the polysilicon layer has a thickness different from a thickness of the second insulating film after the polysilicon layer has been etched.
In accordance with the above-mentioned method, a thickness of a sidewall of the cylindrical accumulation electrode is dependent on a thickness of the second insulating film having remained on an inner surface of the first insulating film. A thickness of a bottom of the cylindrical accumulation electrode is dependent on a condition for etching the polysilicon layer. Thus, the thicknesses of the sidewall and the bottom in the cylindrical accumulation electrode are determined in accordance with different factors, and hence, a thickness of the sidewall is not always equal to a thickness of the bottom unlike the conventional cylindrical accumulation electrode. It is possible to independently determine thicknesses of the sidewall and the bottom of the cylindrical accumulation electrode.
It is preferable that the polysilicon layer is etched in the step (f) so that the polysilicon layer has a thickness greater than a thickness of the second insulating film after the polysilicon layer has been etched.
By designing the bottom to have a greater thickness than a thickness of the sidewall, much reduction in probability in fall-down of a sidewall is ensured, and structural stability in a sidewall of a cylindrical accumulation electrode is enhanced. In addition, it would be possible to accomplish a semiconductor memory device capable of readily increasing a capacity thereof.
For instance, the step (c) may be designed to include the steps of (c1) filling the hole with polysilicon, and (c2) etching back the polysilicon so that an upper surface of the polysilicon is located lower than an upper surface of the first insulating film.
The simplest condition for etching the polysilicon is to control time for etching the polysilicon such that the polysilicon has a thickness greater than a thickness of the second insulating film after the polysilicon has been etched.
It is preferable that the polysilicon is etched to have such a taper that a thickness of the polysilicon is greater at a location closer to a bottom thereof.
For instance, the polysilicon may be etched so that the polysilicon has a straight outer surface and a tapered inner surface.
By designing a thickness of the polysilicon in the above-mentioned manner, a thickness at a lowermost portion of the sidewall is maximum. Hence, it is possible to make a portion through which the sidewall is connected to the bottom greater in size, and hence, increase strength of a connection between the sidewall and the bottom. As a result, it is possible to eliminate structural instability in a cylinder by which the sidewall may be fallen down at a portion through which the sidewall is connected to the bottom.
It is preferable that the method further includes the step of removing the first insulating film so that the lower interlayer insulating film entirely appears.
In comparison with a structure in which an insulating film remains on the lower interlayer insulating film, it is possible to increase an area of an outer surface of the sidewall, as a capacity film, ensuring an increase in a capacity.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
When an area of a memory cell is to be reduced, a sidewall of a cylinder in a cylindrical accumulation electrode has to be designed to have a smaller thickness in order to prevent reduction in both an inner diameter of a cylinder and a capacity, and further to prevent opposing portions of a cylinder from making contact with each other to thereby prevent an inner surface of the cylinder from not acting as an area for defining a capacity.
In accordance with the present invention, it is possible to independently determine thicknesses of a sidewall and a bottom of a cylindrical accumulation electrode, and hence, design the bottom to have a greater thickness than a thickness of the sidewall. This ensures less risk of fall-down of the sidewall of the cylindrical accumulation electrode, higher structural stability, and higher capacity of the cylindrical accumulation electrode.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.